Senescence Marker Protein-30 (SMP30) is an androgen-independent factor that decreases with aging. We recently characterized SMP30 as a gluconolactonase (GNL) involved in the biosynthetic pathway of vitamin C and established that SMP30 knockout mice could not synthesize vitamin C in vivo. Although mice normally synthesize vitamin C, humans are prevented from doing so by mutations that have altered the gluconolactone oxidase gene during evolution. Even the SMP30/GNL present abundantly in the human liver does not synthesize vitamin C in vivo. To clarify the functions of this SMP30/GNL, we transfected the human SMP30/GNL gene into the human liver carcinoma cell line, Hep G2. The resulting Hep G2/SMP30 cells expressed approximately 10.9-fold more SMP30/GNL than Hep G2/pcDNA3 mock-transfected control cells. Examination of SMP30/GNL's impact on the state of oxidative stress in these cells revealed that formation of the reactive oxygen species (ROS) of mitochondrial and post-mitochondrial fractions from Hep G2/SMP30 cells decreased by a significant 24.0% and 18.1%, respectively, compared to those from Hep G2/pcDNA3 cells. Lipid peroxidation levels in Hep G2/SMP30 cells similarly decreased. Moreover, levels of the antioxidants superoxide dismutase (SOD) and glutathione (GSH) in Hep G2/SMP30 cells were a significant 42.6% and 62.4% lower than those in Hep G2/pcDNA3 cells, respectively. Thus, over-expression of SMP30/GNL in Hep G2 cells contributed to a decrease of ROS formation accompanied by decreases of lipid peroxidation, SOD activity and GSH levels.